1. Field of the Invention
The present invention relates to an electronic driving circuit for a bistable actuator activated by piezoelectric elements, particularly suitable for use in residual-current devices such as for example residual-current circuit breakers; the electronic driving circuit according to the invention will be described with particular reference to a residual-current circuit breaker without intending in any way to limit its scope of application.
2. Description of the Related Art
It is known that a residual-current circuit breaker is an automatic circuit breaker for AC circuits which opens when the vector sum of the currents in the conductors of the circuit, which is zero in normal conditions, exceeds a preset value.
Residual-current circuit breakers are meant to prevent metallic parts, such as the enclosures of appliances and the metallic masses of the protected area, connected to an earth system of appropriate resistance, from becoming live ("indirect protection") and, if sufficiently sensitive and fast-acting, can in certain conditions also provide protection against contact with parts which are normally live ("direct protection"). The main characteristic of residual-current circuit breakers is that they achieve extremely short circuit opening and closing times.
A residual-current circuit breaker is shown schematically in FIG. 1. The circuit breaker comprises a current sensor 1 for detecting the differential fault current and which is generally constituted by a toroidal core made of magnetic material. Neutral 2 and the phase 3 of an electric line pass through the core so as to constitute the primary windings of a step-up transformer and are then connected to a system with its loads designated by the reference numeral 10. Secondary winding 4 across which a voltage is generated when the fault current is present also passes through the core. An electronic coupling circuit 5 present between the current sensor 1 and an actuator 6, is supplied by the voltage generated across the secondary winding 4 and drives a release and disengagement mechanism 7 for opening contacts 8 and 9 of the circuit if an earth fault current is present.
Conventional actuators used in residual-current circuit breakers are of the electromagnetic type (electromagnetic relay of the demagnetization type) and have several drawbacks. First they are sensitive to magnetic fields and this alters their operation. For example, the presence of an external magnetic field can produce an unwanted and unwarranted actuation of the actuator. An external magnetic field might also alter the magnetization of the electromagnetic relay, accordingly altering the sensitivity of the residual-current circuit breaker. Moreover, conventional actuators are sensitive to impacts and vibrations and have a complicated mechanical construction which leads to high cost.
In order to obviate all of these drawbacks, bistable actuators activated by piezoelectric elements have been studied. However these devices need to be excited with a voltage pulse which has specific characteristics, in terms of amplitude and rise time, that are substantially different from the pulse supplied by currently available electronic devices for coupling the sensor and the actuator, normally used with electromagnetic relays. Moreover, conventional driving circuits coupled to conventional actuators convert available energy, coupled by the secondary winding of the current sensor 1, mostly in terms of current, not in terms of voltage as required by piezoelectric element-activated actuators.